A hydraulic control system of an automatic power transmission for an automotive vehicle includes, in addition to a pressure regulator for producing a main control or line pressure, a transmission throttle valve which is adapted to produce a throttle pressure continuously variable with load on the engine with which the transmission is used together in an automotive vehicle. Among the transmission throttle valves presently in practical use, there is a throttle valve which is responsive to the movement of the accelerator pedal of a vehicle for producing a throttle pressure variable with the degree to which the throttle valve provided in the carburetor of the engine is open. A transmission throttle valve of this nature usually comprises a valve plunger connected to or engaged by the accelerator pedal through a mechanical linkage provided therebetween and a valve spool engagable with the valve plunger through a spring provided between the valve spool and plunger. The valve spool is arranged between a line pressure circuit communicating with the pressure regulator and a fluid discharge circuit open downstream to a fluid reservoir and is operative to discharge an excess of fluid pressure through the fluid discharge circuit when in producing the throttle pressure on the basis of the line pressure supplied to the transmission throttle valve through the line pressure circuit. In the presence of a line pressure in the line pressure circuit, the valve spool is subjected at one end thereof to the force of the spring and at the other end thereof to a fluid pressure which is developed by the valve spool attempting to attain a certain equilibrium position in which the force resulting from the fluid pressure urging the valve spool against the force of the spring is substantially equally balanced with the force of the spring. The mechanical linkage provided between the valve plunger and the accelerator pedal is arranged so that the valve plunger is continuously movable with the accelerator pedal. As the accelerator pedal is depressed from the released position thereof, the valve plunger is axially moved toward the valve spool and compress the spring provided between the valve spool and plunger. The force of the spring exerted on the valve spool is thus continuously variable with the distance of movement of the accelerator pedal and is therefore representative of the opening degree of the carburetor throttle valve. In other words, the throttle pressure developed by the transmission throttle valve is a function of the opening degree of the carburetor throttle valve and is variable as the accelerator pedal is moved between the released position and a fully depressed or kickdown position thereof. Usually, a transmission throttle valve having these functions is arranged so that the throttle pressure is reduced to zero level in response to the accelerator pedal in the released position and augmented to a peak value close to the level of the line pressure when the accelerator pedal is depressed all the way down.
On the other hand, the pressure regulator valve provided in the hydraulic control system is arranged to be responsive to the throttle pressure thus developed by the transmission throttle valve so that the line pressure to be developed by the pressure regulator is also variable with the opening degree of the carburetor throttle valve. When the throttle pressure is maintained at zero with the accelerator pedal held in the released position, the pressure regulator produces a line pressure which is reduced to a certain minimum value. As the accelerator pedal is depressed and the throttle pressure developed by the transmission throttle valve increases, the line pressure delivered from the pressure regulator increases until the line pressure reaches the above mentioned peak value when the accelerator pedal is fully depressed.
In the event a failure is brought about in the mechanical linkage providing connection or engagement between the accelerator pedal and the valve plunger of the transmission throttle valve, the valve plunger becomes free to move independently of the accelerator pedal and as a consequence the spring provided between the valve plunger and the valve spool of the transmission throttle valve is disabled from imparting a force on the valve spool. When this occurs, the throttle pressure developed by the transmission throttle valve is reduced to zero level and as a consequence the line pressure developed by the pressure regulator drops to the above mentioned minimum level in spite of the fact that the engine is still in operation. If the vehicle must be driven to a repair shop under these conditions without being towed or otherwise aided by another vehicle, the fluid operated frictional units provided in the transmission mechanism are held operative by the line pressure which is reduced to the minimum value as above mentioned. The forces applied to the frictional units by such a line pressure are, however, far short of the loads required of the units so that the frictional units tend to produce undue slips between the friction elements thereof and will in the worst cases invite damages by heat.
With a view to solving these problems, it has been proposed and put into practice to increase the throttle pressure to the level of the line pressure in response to a failure brought about in the mechanical linkage between the transmission throttle valve and the accelerator pedal of the vehicle. A known transmission throttle valve adapted to achieve this function has incorporated therein or is combined therewith a failsafe valve which is arranged to isolate the transmission throttle valve from the previously mentioned fluid discharge circuit in response to the occurrence of a failure in the mechanical linkage. Provision of such an extra valve in or with a transmission throttle valve necessitates the use of a disproportionately large number of additional members and elements as well as a space for accommdating the movement of the failsafe valve and results in an increase in the number of the component parts of the transmission throttle valve and the mechanical linkage between the transmission throttle valve and the accelerator pedal of a vehicle and accordingly in an increase in the steps for the assemblage of a power transmission and the installation of the power transmission into an automotive vehicle. This inevitably gives rise to an increase in the time and labor for the manufacture of an automotive vehicle and accordingly to an increase in the production cost of the vehicle. The present invention contemplates provision of a transmission throttle valve which is not only operable for producing a throttle pressure as in ordinary transmission throttle valves but is capable of functioning in itself as a failsafe valve in the event a failure is invited in the mechanical linkage providing engagement or connection between the transmission throttle valve and the accelerator pedal of an automotive vehicle. Furthermore, the transmission throttle valve proposed by the present invention can be easily modified, by preference, for being capable of coping with kickdown condition of an automotive vehicle so that the shift valves incorporated in the hydraulic control system are biased to make upshifts when the accelerator pedal of the vehicle is depressed throughout the distance of full or nearly full stroke of the pedal. When such modification is made in a hydraulic transmission control system including the transmission throttle valve according to the present invention, not only a failsafe valve but an extra kickdown valve need not be provided in the control system and will further enhance the compactness and production economy of an automatic power transmission.